Chemical Energetics - Entropy

In this post, I will be discussing about Entropy. In short, the term "Entropy" can be described as the extent of disorder of a system. It can be measured by the randomness of the system. However, it is quite difficult to distinguish what is random and what isn't as random?

Introduction

Hence, if there are more ways the particles can be arranged (as seen above), the more disordered and naturally, the system is more randomed. Since, the units of entropy is J mol^-1 K^-1, we would expect a energetics way to describe entropy (Just like potential energy describes the electrostatic attractive forces and etc).

Entropy, thus, can be seen as how spread out the energy possessed by the system (is it localised or not). To fully appreciate this idea, it is important for someone to know the concept of molecular orbital theory, which is beyond the scoop of this discussion.

Therefore, the main tenet of this post will be using the description that entropy can be determine by the number of ways the the particles can be arranged. Thus, using this as a means to account for how the following facts can cause entropy to increase.

(a) Change from Solid to liquid (change from liquid to gas).

The idea of solid is that the volume is fix and there is a definite structure. Hence, the arrangement of the particles is fixed thus very little different distribution of the arrangement of particles since they aren't mobile.

In liquids, the volume is definite, however the structure isn't. Hence, this helps to conceptualise that the arrangement of particles in the liquid state is less rigid; there is a more varied arrangements of the particles because there is some amount of mobility.

Lastly, the volume of a gas is not definite (since gas can be compressed), and there is no logical means to describe its shape, hence, there must be numerous arrangementa of the particles in gasous state, since they will have the greatest mobility.

Hence, entropy increases when we change the physical state of a substance from solid to liquid (and from liquid to gas).

(b) Increasing in Temperature

Generally increasing temperature will definitely cause the particles to gain mobility, since the kinetic energy which the particle possess increase and it would be more abled to overcome attractive forces that restrict its movement.

Hence, increasing temperature will certain aid to create a more varied distribution of the particles, and that causes entropy of the system to increase. However, the quantitative extent which entropy increase is beyond the scoop of this discussion.

(c) Mixing

The idea of mixing is when a solute is added to a solvent. In particle terms, in order to dissolve the solute in a solvent, the solute particles must be surrounded by the solvent particles.

For example, to dissolve glucose (solute) into water (solvent), the water molecules must surround the glucose molecules. Thus, to have water molecules to surround the glucose molecules favourably, water molecules must be able to form hydrogen bonding with the glucose molecules.

Needless to say, when we initially only have pure solute and pure solvent, the particles are concentrated amongst themselves. When mixing is done, it would result in a greater variety in the arrangement of the particles since they ceased to be concentrated amongst themselves (because of the concept of mixing). Hence, entropy increases when we mix substances together.

(d) Production of (Gasous) more particles

The production many small molecules can generally to cause an increase in entropy, which sometimes is the reason for causing endothermic reactions to be spontaneous.

However, it because of an increase in the total number of particles (after a chemical reaction) which results in entropy increase. This is because this creates a larger number of possible arrangements of the particles, hence an entropy increase.

In addition, an increase in number of gasous particles after a reaction, will most definitely cause entropy to increase, since gas particles are highly disordered, it will add to randomness of the system.

Therefore, it is also good to check the physical state which the products formed exist in, since formation of more solid products will not be expect to lead to an increase in entropy.

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Article written by Kwok YL 2007 . Edited in June 2009.

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